Shock wave techniques have been used extensively to collect information on the high-pressure equation-of-state (EOS) of materials. However, most high-pressure EOS data have been obtained from shock compression which represents the response of a material along its principal Hugoniot. The need for accurate off-Hugoniot measurements has compelled the development of several experimental approaches to produce well-controlled continuous or ramp loading of condensed matter. Ramp loading of all materials and solids, in particular, generally produces thermodynamic states close to an isentrope since irreversible effects produced by viscoplastic and plastic work are usually small. This technique is often referred to as an isentropic compression experiment (ICE).
To generate a magnetic pressure of 100 gigapascals (GPa) on the surface of a conductor requires a 500 T magnetic field. Such a field is generated by a linear current density (on the conductor's surface) of 400 MA/m. The use of such current densities to generate magnetic pressures to drive material-physics experiments is described in the literature. See D. B. Reisman et al., J. Appl. Phys. 89, 1625 (2001); C. A. Hall et al., Rev. Sci. Instrum. 72, 3587 (2001); and M. D. Knudson, Shock Compression of Condensed Matter—2011, AIP Conf. Proc. 1426, 35 (2012). The technique has been used over the past 15 years for a variety of material studies. Currently, the refurbished Z accelerator (also referred to as ZR) produces tailored current pulses to drive samples quasi-isentropically to pressures as high as 500 GPa over timescales as long as 1 microsecond. See J.-P. Davis et al., Physics of Plasmas 12, 056310 (2005); and J.-P. Davis et al., J. of Appl. Phys. 116, 204903 (2014). In this technique, planar load samples (6-10 mm in diameter by 0.5-1.5 mm thick) are mounted on a flat anode plate of either aluminum or copper. A direct short between the parallel anode and cathode plates allows a ˜20 MA, ˜100-300 ns risetime current pulse to flow from one plate to the other, which generates a planar time-varying magnetic field between the conductors. The resulting large magnetic pressure launches a high-pressure ramp wave into the anode conductor and hence into the planar sample. A smooth, shockless compression is achieved with comparatively low compression strain rates of about 106/s.
However, because the ZR accelerator is a large experimental facility designed to accommodate multiple scientific program needs, research for ICE studies is hampered by limited available machine time, considerable operational constraints, and expense. Further, although the pulsed-power technique has proven to be quite productive at large scale, several key issues must be examined thoroughly to extend the ICE method to different pulsed-power driver configurations. To expand the use of pulsed-power techniques for ICE studies, a compact pulsed-power generator, referred to as Veloce, was developed specifically for isentropic and shock compression experiments. See T. Ao et al., Rev. Sci. Instrum. 79, 013903 (2008), which is incorporated herein by reference. Veloce is a low inductance generator based on a stripline design where no oil, water, or vacuum is used for insulation, thus making it much easier to operate and maintain. The generator occupies a 3.6×5.5 m2 area and delivers up to 3 MA of current rapidly over ˜440-530 ns into an inductive stripline load where significant magnetic pressures can be produced. The magnetic pressure on Veloce can be used either to drive ramp pressure waves (5-20 GPa) into material samples or to launch relatively thick flyer plates (1-2 mm) to velocities of 1-3 km/s. However, the generator can only produce sub-megabar pressures. For most materials, the Hugoniot and isentrope diverge near a megabar, which is a pressure regime useful for equation-of-state studies. Further, because the Veloce generator uses a parallel plate transmission line, it has limited pulse shaping capability and, therefore, limited pressure ramping flexibility.
Therefore, a need remains for an accelerator that can produce variable pulse shapes with shorter rise times in order to maximize ramp wave propagation distances before shock formation occurs, thereby enabling the study of larger samples.